Astronomy. Astrophysics. Radio jet emission from GeV-emitting narrow-line Seyfert 1 galaxies,

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1 A&A 575, A55 (25) DOI:.5/4-636/24258 c ESO 25 Astronomy & Astrophysics Radio jet emission from GeV-emitting narrow-line Seyfert galaxies, E. Angelakis, L. Fuhrmann, N. Marchili 2, L. Foschini 3, I. Myserlis, V. Karamanavis,S.Komossa,D.Blinov 4,5, T. P. Krichbaum, A. Sievers 6, H. Ungerechts 6, and J. A. Zensus Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 532 Bonn, Germany eangelaki@mpifr-bonn.mpg.de 2 Dipartimento di Fisica e Astronomia, Università di Padova, via Marzolo 8, 353 Padova, Italy 3 INAF Osservatorio Astronomico di Brera, via E. Bianchi 46, 2387 Merate (LC), Italy 4 Department of Physics and Institute of Theoretical & Computational Physics, University of Crete, 7 3 Heraklion, Greece 5 Astronomical Institute, St. Petersburg State University,Universitetsky pr. 28, Petrodvoretz, 9854 St. Petersburg, Russia 6 Instituto de Radio Astronomía Milimétrica, Avenida Divina Pastora 7, Local 2, 82 Granada, Spain Received 29 September 24 / Accepted 6 December 24 ABSTRACT Context. With the current study we aim at understanding the properties of radio emission and the assumed jet from four radio-loud and γ-ray-loud narrow-line Seyfert galaxies that have been detected by Fermi. These are Seyfert galaxies with emission lines at the low end of the FWHM distribution. Aims. The ultimate goal is twofold: first we investigate whether a relativistic jet is operating at the source producing the radio output, and second, we quantify the jet characteristics to understand possible similarities with and differences from the jets found in typical blazars. Methods. We relied on the most systematic monitoring of radio-loud and γ-ray-detected narrow-line Seyfert galaxies in the cm and mm radio bands conducted with the Effelsberg m and IRAM 3 m telescopes. It covers the longest time-baselines and the most radio frequencies to date. This dataset of multi-wavelength, long-term radio light-curves was analysed from several perspectives. We developed a novel algorithm to extract sensible variability parameters (mainly amplitudes and time scales) that were then used to compute variability brightness temperatures and the corresponding Doppler factors. The jet powers were computed from the light curves to estimate the energy output and compare it with that of typical blazars. The dynamics of radio spectral energy distributions were examined to understand the mechanism causing the variability. Results. The length of the available light curves for three of the four sources in the sample allowed a firm understanding of the general behaviour of the sources. They all display intensive variability that appears to be occurring at a pace rather faster than what is commonly seen in blazars. The flaring events become progressively more prominent as the frequency increases and show intensive spectral evolution that is indicative of shock evolution. The variability brightness temperatures and the associated Doppler factors are moderate, implying a mildly relativistic jet. The computed jet powers show very energetic flows. The radio polarisation in one case clearly implies a quiescent jet underlying recursive flaring activity. Finally, in one case, the sudden disappearance of a γ-ray flare below some critical frequency in our band needs a more detailed investigation of the possible mechanism causing the evolution of broadband events. Conclusions. Despite the generally lower flux densities, the sources appear to show all typical characteristics seen in blazars that are powered by relativistic jets, such as intensive variability, spectral evolution across the different bands following evolutionary paths explained by travelling shocks, and Doppler factors indicating mildly relativistic speeds. Key words. galaxies: active gamma rays: galaxies galaxies: jets galaxies: Seyfert radio continuum: galaxies. Introduction Seyfert galaxies were first identified as a distinct class of extragalactic systems by Seyfert (943), who studied the nuclear emission of six extragalactic nebulae with high excitation nuclear emission lines superposed on a normal G-type spectrum. The lines appeared to be broadened, reaching widths of about 85 km s. Seyfert also noted that the maximum width of the Balmer emission lines increased with the luminosity of Appendices A and B are available in electronic form at Data displayed in Figs. 2 and 3 (Table 7 is an example) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( ) orvia the nucleus as well as the ratio between the light from the nucleus and the total light of the object, setting the scene for the class of Seyfert galaxies that in summary show, bright, star-like nucleus, and broad emission lines that cover a wide range of ionisation states. Khachikian & Weedman (974) much later classified Seyfert galaxies into types and 2 depending on whether the Hβ lines (Balmer series) are broader than the forbidden ones, or of approximately the same width. Typically, the width of forbidden lines in both classes are of the order of narrow emission lines, that is, of about 3 8 km s, while the width of broad emission lines including the hydrogen Balmer lines is about 6 km s (Osterbrock 984). Davidson & Kinman (978) found that MRK 359 appeared, to be lying at the low end of the line-width distribution and to have Hβ and forbidden lines of comparable widths that Article published by EDP Sciences A55, page of 22

2 A&A 575, A55 (25) were similar to those in Seyfert 2 galaxies ( 3 km s ); furthermore, MRK 359 showed strong featureless continuum, and strong high-ionisation lines (e.g. [Fe vii]and[fex]); these properties are common in Seyfert galaxies, which led to the definition of yet another sub-class of active galactic nuclei (AGN: the narrow-line Seyfert hereafter NLSy galaxies (Gaskell 984; Osterbrock & Pogge 985; Osterbrock & Dahari 983). Koski (978) and Phillips (978) noted that MRK42 showed a similar behaviour. Osterbrock & Pogge (985) studied eight such sources and concluded that they are characterised by (a) unusually narrow Hβ lines; (b) strong Fe ii emission; (c) normal luminosities; and (d) Hβ slightly weaker than in typical Seyfert galaxies. Conventionally, today sources are categorised as NLSy if they show (a) a narrow width of the broad Balmer emission line with a FWHM(Hβ) < 2 km s ; and (b) weak forbidden lines with [O iii]λ57/hβ <3(Osterbrock & Pogge 985; Goodrich 989; Zhou et al. 26). The first attempts to investigate the radio properties of these systems were undertaken by Ulvestad et al. (995), among others, who studied seven NLSys with the Very Long Array (VLA) in A configuration at 5 GHz. They found that the radio power was moderate ( 2 23 WHz ), the emission is compact (<3 pc), and in the two of three cases where radio axes could be found and high optical polarisation was detected, the radio axes were oriented perpendicularly to the electric vector position angle (EVPA); the third one showed EVPA nearly parallel to the radio axis. Later, Moran (2) also studied 24 NLSys with the VLA in A configuration to confirm that most of the sources were unresolved and that they showed relatively steep spectra. Stepanian et al. (23) investigated 26 NLSy galaxies and found that only 9 were detected in the FIRST catalogue (White et al. 997), and all were radio-quiet. Komossa et al. (26) studied 28 NLSys in a dedicated search for radio-loud (RL) NLSys and concluded, among others, that (a) their morphology is similar to compact steep-spectrum sources (CSS; Gallo et al. 26, discussed PKS as an example of NLSy that was also CSS); (b) the radio-loudness R defined as the ratio of the 6 cm flux to the optical flux at 44 Å (Kellermann et al. 989) is distributed smoothly up to the critical value of and covers about four orders of magnitude; (c) almost 7% of the NLSy galaxies are formally RL, but only 2.5% of them exceed a radio index R > ; (d) most RL NLSy are CSS, accreting close to or above the Eddington luminosity, L Edd ; (e) their black-hole masses are generally at the upper observed end for NLSy, but are still smaller than what is typically seen in other RL AGNs. Even before the detection of the RL subset of this class at high energies, it was clear that they comprise a special source type for a number of reasons. Yuan et al. (28) studied a sample of 23 NLSys with a radio-loudness exceeding to find that these RL AGNs may be powered by black holes of moderate masses ( 6 7 M ) accreting at high rates and that they show a variety of radio properties reminiscent of blazars. Komossa et al. (26) pointed out that NLSys provide an excellent probe for studying the physics scaling towards lower blackhole masses given their systematically low black-hole masses in the range 6 8 M. Despite the fundamental differences between RL NLSys and blazars in terms of black-hole masses and accretion disc luminosities (.2.9 L Edd ) their jets seem to be behaving similarly and share the same properties (see also Foschini 22b). That alone has implications on the energy production and dissipation at different scales. According to Komossa et al. (26), an important question arising from the behaviour of these systems is whether or not the black-hole mass and radio-loudness are related and especially whether there is a limiting black-hole mass above which objects are preferentially RL, and whether or not RL and radio-quiet galaxies show the same spread in their black-hole masses. On this ground, Komossa (28) argued that the study of NLSys needs to (a) include the radio and infrared properties of NLSy galaxies when correlation analyses are performed; (b) determine the sufficient and necessary conditions for the onset of NLSy activity; and (c) the investigation of whether the low blackhole mass is enough to explain the typically observed NLSy characteristics. The first report for the detection of γ-ray emission from a source classified as an RL NLSy was given by Abdo et al. (29a) andfoschini et al. (2), who discussed the detection of significant GeV emission by the Fermi/LAT instrument from PMN J Already in its first year of operations, Fermi/LAT detected a total of four NLSys (Abdo et al. 29c). As of today, there are seven RL NLSys detected in the MeV GeV energy bands five of which with high significance (TS > 25, see also D Ammando et al. 23b, Foschini et al. 25). A thorough review of the recent discoveries is given by Foschini (22b). The multi-wavelength campaigns that followed these discoveries (e.g. Abdo et al. 29b,a; Giroletti et al. 2; Foschini et al. 22; Fuhrmann et al. 2) and the study of subsequently discovered NLSys (Abdo et al. 29c) showed a clear blazar-like behaviour, indicating the existence of a relativistic jet viewed at small angles. Despite the research activity that their high-energy detection has motivated, several questions remain open. Especially the exact nature and properties of NLSys and the properties of the jet that seems to be present. Particularly in the radio regime, where a jet is expected to be dominant, the studies conducted so far have mostly been based on non-simultaneous datasets. In the current work we quantify some of the properties of the assumed radio jet emission. The motivation for this is to compare them with those seen in other classes of AGNs especially blazars. To do this, we studied the radio behaviour of four RL NLSys detected in γ rays by Fermi/LAT through regular multi-band, single-dish radio monitoring with the Effelsberg m and IRAM 3 m telescopes. We focus on the variability properties especially brightness temperatures and variability Doppler factors, which will (in a later publication) be combined with long-baseline radio interferometric measurements to estimate the viewing angles and Lorentz factors (Fuhrmann et al., in prep.). The energetics of the observed radio outbursts and the variability patterns of the broadband radio spectra are seen as indicators of the variability and emission mechanism at play. Very early results have been presented in several other studies, including D Ammando et al. (22), Fuhrmann et al. (2), and Foschini et al. (22). Here we present the longest-term, multi-frequency radio monitoring datasets of the known γ-ray-detected and radioloud NLSys to date. As an example, the 4.85 GHz lightcurve lengths, range between longer than.5 years and longer than 5 years. The monitoring was conducted at eight different frequencies for the two brightest sources and at six frequencies for the faintest. The paper is structured as follows: in Sect. 2 we review some archival information about the sources in our sample that is necessary for the following discussion. After describing the observing methods in Sect. 3, we continue in Sect. 4 with the phenomenological description of the obtained light curves. The light curves discussed there comprise the basis for the flare A55, page 2 of 22

3 E. Angelakis et al.: Radio jet emission from GeV-emitting NLSys Table. The four monitored NLSys with their observed positions, redshift, and classification. Source ID Survey name RA Dec z Class (hh:mm:ss.s) (dd:mm:ss.s) J B B 3:24: ::45..6 a NLSy.629 b J SBS :49:58. +5:8: c NLSy 2 J PMN J :48:57.3 +:22: c NLSy 3 J PKS :5:6.5 +3:26: c NLSy 4 References. (a) Marcha et al. (996); (b) Zhou et al. (27); (c) Marcha et al. (996); () Zhou et al. (27); (2) Greene & Ho (25); (3) Zhou et al. (23); (4) Zhou et al. (26). decomposition method presented in Sect. 5. Subsequently, we continue in the frequency domain by studying the radio spectra in Sect. 6. In Sect. 7 we compute the radio powers for the 4.6 GHz datasets, while in Sect. 8 we discuss our polarisation measurements. Summarising comments are discussed in Sect. 8, where we describe the general connection of all our findings. The main findings of our study are presented in Sect., which concludes the paper. Throughout the paper we assume a ΛCDM cosmology with H = 7 km s Mpc and Ω Λ =.73 (Komatsu et al. 2). 2. The sample: what is already known We here investigate a sample of four sources, which are:. classified as NLSys; 2. radio-loud; 3. detected at γ rays by the Fermi/LAT; and 4. satisfy certain observational constrains such as declination limits and a sufficiently high flux density to enable quality measurements at the Effelsberg (>. Jy) and IRAM telescopes (>.3 Jy). We briefly summarise what is already known of these sources. Table summarises their positions, redshifts, and classifications. 2.. J (H ) The source J is of special interest mostly because of the morphology of its host galaxy and the estimated black-hole mass. From two HST archival images (each of 2 s exposure), Zhou et al. (27) concluded that it is hosted by a one-armed spiral galaxy. The authors showed that its radio and X-ray emission can be explained by jet synchrotron radiation, while the infrared and optical light is dominated by thermal emission from a Seyfert nucleus. Antón et al. (28) conducted B- andr-band observations with the Nordic Optical Telescope (NOT). They suggested that its host resembles the morphology seen in the inner parts of Arp found by Charmandaris & Appleton (996), implying that J may be a merger remnant. Following the method discussed by Greene & Ho (25), they estimated the central black-hole mass to be 7 M ; this value lies in the overlapping region between the black-hole mass distributions for NLSys and blazars and is similar to the value published earlier by Zhou et al. (27). It is worth mentioning that León Tavares et al. (24) conducted a detailed multi-filter host-galaxy study. Although their results were not conclusive as to whether the host is a spiral galaxy, using a 2D Fourier analysis they found evidences for a ring structure likely caused by a recent merger episode in consistency with Antón et al. (28) findings. Although the only case of an RL, Fermi-detected NLSy for which the host is well resolved, J324+34, challenges the assumptions that powerful relativistic jets form only in giant elliptical galaxies (e.g. Böttcher & Dermer 22; Sikora et al. 27). Abdo et al. (29c) presented a thorough study including spectral energy distribution (SED) fits. The presence of a jet was already apparent in these studies; but in an episodic manner and not as a fully developed constantly broadband-emitting jet. The computed jet power placed the source in the BL Lac range (see jet power computations in Sect. 7). It is worth noting that they computed accretion rates that reach extreme values of up to 9% of the Eddington luminosity, yet another peculiar property of the RL, γ-ray-loud NLSys J (SBS ) J was identified as an NLSy galaxy by Zhou et al. (25). They found clear evidence for emission from a relativistic jet and a stellar component. Additionally, the emission lines show characteristics that classify it as a typical NLSy (FWHM(Hβ) 7 km s,[oiii]λ Hβ and strong Fe ii emission). The fact that the source was previously classified as a BL Lac object (Arp et al. 979) makes the case especially interesting, possibly providing a handle on the link between BL Lac objects and NLSys. Yuan et al. (28) included the source in a very thorough study of a sample of 23 RL NLSy galaxies and reported a blackhole mass of about 7.4 M. On the basis of the detected significant optical polarisation, they claimed that it has a jet. Finally, they found that during the high-energy states the optical continuum is featureless, while at low states strong emission lines become obvious; this suggests that the source maybe a transition between a quasar and a BL Lac that displays its latter character only at flaring states. Foschini (2b) was the first to report its detection at γ rays. Later, D Ammando et al. (22) discussed a flaring episode around June July 2. The multi-frequency datasets collected during the campaigns following the detection are partly discussed in D Ammando et al. (23c). Using the Very Long BaselineArray(VLBA)at5,8.4and5GHzD Ammando et al. (22) resolved the otherwise compact source to reveal a corejet structure. The feature attributed to the jet shows a steep spectrum. The core has been decomposed into two compact components. They discussed the detection of an apparent speed of about 8 c, indicating a relativistic jet. In summary, the power output (isotropic γ-ray luminosity) of 48 erg s on a daily scale (similar to that of luminous flat-spectrum radio quasars, FSRQs), the apparent superluminal motions and the radio variability accompanied by spectral evolution indicate blazar-like relativistic jet. D Ammando et al. (22) also considered the suggestion of Ghisellini & Tavecchio (28) and Ghisellini et al. (2) that the transition between FSRQs and BL Lac can be interpreted in terms of different accretion rates. They found that the position A55, page 3 of 22

4 A&A 575, A55 (25) of the source in a γ-ray photon index against luminosity plot (Γ γ L γ ) also places it in the typical blazar territory J (PMN J948+22) J shows the usual characteristics of an NLSy optical spectrum (Williams et al. 22) with a FWHM(Hβ) of 5± 55 km s,oiii/hβ <3, and strong optical F ii emission (Zhou et al. 23). Although NLSys are usually radio quiet (e.g. Ulvestad et al. 995; Komossa et al. 26), J was found to be the first very radio-loud NLSy with a R > 3 (Zhou et al. 23). In addition, Zhou et al. (23) reported an inverted radio spectrum and high brightness temperatures, which strongly supports that there is a relativistic, Doppler-boosted jet seen at small viewing angles. The source has also been the first NLSy detected at high-energy γ rays by Fermi/LAT during its first months of operation. Fermi/LAT confirmed the relativistic jet emission from this source and established NLSys as a new class of γ-ray emitting AGNs (Foschini et al. 2; Abdo et al. 29a,b). At pc scales, the source appears as one-sided VLBI structure dominated by a compact (<55 μas), bright central component (Doi et al. 26; Giroletti et al. 2). Kpc-scale radio emission has also been found by Doietal.(22) with a two-sided extension of the core and a northern extent of 52 kpc in projected distance. From these studies a relativistically boosted jet with Doppler factors D > (e.g.d > , Doi et al. 26) has been inferred, in agreement with Doppler boosting inferred from previous variability studies (e.g. Fuhrmann et al. 2). This NLSy is variable across the whole electromagnetic spectrum. Intense multi-wavelength campaigns performed over the past years revealed continuous activity with an exceptional outburst in 2 (Foschini et al. 2) that occurred at all spectral bands. Together with detailed SED modelling and the detection of polarised emission, these findings confirm the powerful relativistic jet in the source seen at small viewing angles J (PKS 52+36) Based on its optical characteristics (FWHM(Hβ) = 82 ± 3 km s,[oiii]/hβ., and a strong optical Fe ii bump), this source was classified as NLSy (e.g. Yuan et al. 28). It exhibits one of the highest radio-loudness parameters among NLSys (R = 549, see also Sect. 2.5). Early VLBI observations revealed a compact source (marginally resolved at milli-arcsecond scales) with an inverted radio spectral index (Dallacasa et al. 998). Abdo et al. (29c) reported the first detection of γ-ray emission from the source by Fermi/LAT. From SED modelling the authors inferred jet powers similar to those in J and those typically observed in powerful quasars. A detailed multiwavelength campaign carried out between 28 and 22 did not reveal any significant variability at γ rays, in contrast to prominent flux density and spectral variability seen in the radio regime (D Ammando et al. 23a). The 5 GHz VLBI imaging showed a one-sided core-jet structure on pc scales. No significant proper motion of jet components was detected. The radio variability and VLBI findings together with an inferred high apparent isotropic γ-ray luminosity strongly support a relativistic, Doppler-boosted jet in this case as well Radio-loudness Of the four sources in our study, J948+22, J849+58, and J are very RL, with R.4 = Table 2. Radio-loudness of the sources in our sample. Source Radio index (a) Reference J R 5 = 38 7 () Based on HST optical flux J R.4 = (2) Based on USNO and GSC catalogues (note the variable optical flux) J R.4 = 445 (3) Based on SDSS optical flux J R.4 = 549 (3)... Notes. The moderate variability cannot cause remarkable changes in the R indices, making the published values representative enough. (a) With R 5 = S 5 GHz /S B, R.4 = S.4 GHz /S 44A Kellermann s radio index. Under the assumptions of Kellermann et al.: R.4 =.9R 5. References. () Zhou et al. (27); (2) Komossa et al. (26); (3) Yuan et al. (28). (Komossa et al. 26), 445 and 549 (Yuan et al. 28), respectively; where R is the radio index according to Kellermann et al. (989). J is only mildly RL with R 5 = 38 7 (Zhou et al. 27). We note that each value of R is uncertain by a factor of a few to, reflecting uncertainties in extinction correction, optical host contribution, and variability in the radio or optical bands. The monitoring presented here, reveals variability in the radio bands of up to several ten of percent (see Table 8), but not high enough to modify the previous source classification as RL or radio-quiet. The values of the radio indices reported in the literature are therefore adequate for the scope of this paper and are shown in Table Observations and data reduction The light curves and radio SEDs presented here have been observed in the framework of the F-GAMMA programme (Fuhrmann et al. 27; Angelakis et al. 2). They cover the frequency range from 2.64 to GHz. Below 43.5 GHz the measurements were conducted with eight different heterodyne receivers mounted on the secondary focus of the m Effelsberg telescope. The observations at and GHz were obtained with the 3 m IRAM telescope. In Table 4 their most important operational characteristics are summarised. 3.. Effelsberg observations The Effelsberg station covers the band between 2.64 and 43.5 GHz. The systems at 4.85,.45 and 32 GHz are equipped with multiple feeds allowing differentialmeasurementsmeant to remove atmospheric effects (e.g. emission or absorption fluctuations). The rest are equipped with only single feeds. All receivers have circularly polarised feeds. The observations are made in cross-scanning mode, that is monitoring the telescope response while driving over the source position in two different orthogonal directions (in our case, azimuth and elevation). Necessary post-measurement corrections include the following:. Pointing-offset correction: meant to correct for the power loss caused by possible divergence of the actual source position from that observed by the telescope. This effect is of the order of a few percent. 2. Atmospheric-opacity correction: meant to correct for the attenuation caused by the transmission through the terrestrial atmosphere. This effect can be significant especially at higher frequencies where the atmospheric opacity becomes significantly high. 3. Elevation-dependent gain correction: correcting for sensitivity losses caused by small-scale departures of the primary A55, page 4 of 22

5 Table 3. Flux densities of the standard calibrators used at Effelsberg. E. Angelakis et al.: Radio jet emission from GeV-emitting NLSys Source: 3C 48 3C 6 3C 286 3C 295 NGC 727 a S S S S S S S S Notes. (a) The flux density of NGC 727 is corrected for its extended size at frequencies above.45 GHz. References. The flux densities of the calibrators are taken from Ott et al. (994), Baars et al. (977), Zijlstra et al. (28), and Kraus (priv. comm.). reflector s geometry from that of an ideal paraboloid (as expected for a homology-designed reflector). The magnitude of this effect is constrained to within a few percent. 4. Absolute calibration: converting the measured antenna temperatures to SI units by reference to standard candles, that is, calibrators. The standard candles used for our programme along with the flux densities assumed for them are shown in Table 3. Specifically, in the case of NGC 727, a partial power loss caused by its extended structure relative to the beam size above.45 GHz was corrected for. The five-year mean magnitudes of these effects as they are observed by the F-GAMMA programme are summarised in Table IRAM observations Observations with the IRAM 3 m telescope were made within the F-GAMMA monitoring programme and the more general flux monitoring conducted by IRAM (Institut de Radioastronomie Millimétrique, Ungerechts et al. 998). Data from both programmes are included in this paper. The observations were conducted with the newly installed EMIR receiver (Carter et al. 22) using the 3 and 2 mm bands (each with linear polarisation feeds) tuned to and GHz and the narrow-band continuum backends ( GHz bandwidth) attached. Observations of J and J were performed with cross-scans in azimuth and elevation direction and wobblerswitching along azimuth with a frequency near 2 Hz. Each crossscan was preceded by a calibration scan to obtain instantaneous opacity information (e.g. Mauersberger et al. 989). After data quality control, the sub-scans of each scanning direction were averaged and fitted with Gaussian curves. In the next step, each amplitude was corrected for small remaining pointing offsets and systematic gain-elevation effects. This operation has an effect of about % at GHz and 4% at GHz (mean pointing offsets are.8 for both receivers). The latter correction, and given the IRAM 3 m elevation-dependent gain curves, amounts to < 5%. The conversion to the standard flux density scale was made using frequent observations of primary (Mars, Uranus) and secondary (W3OH, K3-5A, NGC 727) calibrators Error estimates In every operational step the errors were propagated formally assuming Gaussianity. The exact details are discussed in separate papers (Nestoras et al., in prep., for the IRAM observations; Fig.. Observed radio SED of NGC 727 (Zijlstra et al. 28) over the same period of time as the time baseline covered here. All the Effelsberg frequencies from 2.64 up to the highest IRAM frequency of GHz are shown. The filled circles (red and black) denote the measurements. The red symbols mark the Effelsberg measurements that where used in the fit, the red dotted-dashed line is the fitted spectrum. The grey area denotes the σ region around each data point. The agreement of the extrapolated values with those measured by IRAM is better than 3%. Angelakis et al., in prep., for the Effelsberg observations). An indicative empirical measure of the uncertainty in a measurement can be given by the fractional fluctuations seen in sources known to be intrinsically stable, that is to say, the calibrators. The datasets available for these sources cover the entire period of observations and include cumulatively all possible sources of fluctuations. This variability seen in sources intrinsically stable must also be present in the light curves of the targets. In Table 6 we quote the mean flux density at each frequency for each calibrator used at Effelsberg and NGC 727 used at IRAM. Note that the flux density for each such source was computed using the mean calibration factor of each session. There we also show the modulation index defined as m = σ S with σ being the standard deviation and S the mean flux density. m remains at levels of a few percent even at frequencies above 23.5 GHz where the troposphere becomes very disturbing Cross-telescope calibration Because below we discuss the dynamics of radio SEDs observed partly with the Effelsberg and partly with the IRAM 3 m telescope, it is essential to address the cross-telescope calibration accuracy that might introduce artefacts in the observed spectral shapes and their apparent temporal behaviour. An empirical yet reliable measure of its goodness can be provided by NGC 727, which exhibits a flux density high enough to be detected by both instruments with high signal-to-noise ratios (S/N) and repeatedly. NGC 727 has a very well defined and analytically described time-dependent convex spectrum (Zijlstra et al. 28) that at frequencies above roughly GHz can be approximated by a power law of the form S ν..infig. we show the flux densities averaged over the entire period discussed here and for all frequencies. Each circle denotes an average flux density. The red symbols (with ν.45 GHz) are the Effelsberg measurements that were used for the model fit, and the red line is the result of the fit. The grey area is confined by σ of each average flux density. Extrapolating the fitted spectrum (red dotted-dashed line) towards the higher IRAM band A55, page 5 of 22

6 A&A 575, A55 (25) Table 4. Receiver characteristics. ν λ Δν T sys FWHM Feeds Polarisation Aperture efficiency (GHz) (mm) (GHz) (K) ( ) (%) LCP, RCP LCP, RCP LCP, RCP LCP, RCP LCP, RCP LCP ( GHz used) 65 a 29 HLP, VLP ( GHz used) 65 a 6 HLP, VLP 57 Notes. The entries in each column is as follows: ) central frequency; 2) receiver bandwidth; 3) system temperature; 4) sensitivity; 5) full width at half maximum (FWHM); 6) number of available feeds; 7) available polarisation channels; 8) telescope effective area at the corresponding frequency. (a) The values quoted for the IRAM receivers are typical values for the receiver temperatures T rx and not system temperatures, hence they do not include atmospheric contributions, background emission, etc. Table 5. Average fractional effect of each post-measurement correction applied to the data for each observing frequency. Frequency Pointing Opacity Gain correction correction correction (GHz) (%) (%) (%) Effelsberg IRAM Notes. The numbers represent the 5-year average behaviour as it is observed by the F-GAMMA programme. and comparing these values with the measured IRAM 3 m flux densities yields differences better than 3%, specifically, 2.7% at GHz and 2.8% at GHz, indicating a high-quality cross-telescope calibration Analysis methods The current section introduces notions and methods that are used below, even if they are repeated briefly in the corresponding sections. Internal shocks causing variability: one of the aims here is to study the mechanism that may be causing the observed variability. Throughout the text it is assumed that this could well be caused by internal shocks that propagate in the jet, imprinting a specific signature in the radio light curves. In this model (Marscher & Gear 985; Türler et al. 2), the synchrotron selfabsorbed component is undergoing distinct evolutionary stages, each characterised by a different energy-loss mechanism. The followed path then imprints a distinct phenomenology on the radio SEDs, making this model easily quantifiable and testable. It has been argued that the implementation of this scenario in a system with simply a steep-spectrum quiescent jet is enough to reproduce the observed plurality of phenomenologies (Angelakis et al. 22b). The variability brightness temperature: as discussed extensively in Sect. 5, the variability brightness temperature is a measure of the energetics of the associated event. It is computed on the basis of the light travel-time argument and depends on the magnitude of the flux density variation, δs and the time span needed for that variation, δt. The flare decomposition method (Sect. 5) aims at separately estimating exactly those parameters for each event. The variability brightness temperature at the source rest frame in K, is given by T var =.64 δs dl 2 λ2 δt 2 ( + z), () 4 where δs is the increase in flux density (at the observer s frame) in units of Jy, δt is the time span needed for that increase (at the observer s frame) in units of days, d L is the source luminosity distance in units of Mpc, λ is the observing wavelength in units of cm, and z is the source redshift, all measured at the observer s frame. Any excess from an independently computed intrinsic limit is then interpreted in terms of the Doppler-boosting factor D as Tvar D = ( + z) 3+α, (2) T ref where α is the source spectral index with S ν α, T ref is the limiting value of the brightness temperature. The value for the spectral index in the previous equation is chosen to be the mean values in the corresponding sub-band as given in Table. In Sect. 5 we explain that the limiting value is based on the equipartition argument as proposed by Readhead (994). Here we assumed that the emission comes from a single blob. For a continuous jet the index 3 + α should be replaced by 2 + α. A55, page 6 of 22

7 E. Angelakis et al.: Radio jet emission from GeV-emitting NLSys Table 6. Mean flux density and modulation index of the calibrators at each frequency. Source Observable Units C 286 N S (Jy) m (%) C48 N S (Jy) m (%) C6 N S (Jy) m (%) C295 N S (Jy) m (%) NGC 727 N S (Jy) m (%) Table 7. Example of a light-curve file as published online at CDS. The current extract is for J at 4.85 GHz. 4. Radio light curves S Error (Jy) (Jy) The dataset discussed here is the result of the longest-term multifrequency radio monitoring for RL and γ-ray loud NLSys available. It covers the period between early-29 and mid-24. In Figs. 2 and 3 we present the available light curves at all observing frequencies. We only show measurements with a S/N better than 3. All the light curves are also available online at the CDS. Table 7 is an example of an online light-curve file for J at 4.85 GHz. It contains the following information: Column lists the of a measurement, Col. 2 gives the flux density in units of Jy, and Col. 3 lists the associated error. The mean sampling averaged over all frequencies ranges from one measurement every 24 days for J and J (approximately the cadence of the most commonly used calibrator, 3C 286), and 29 days for J55+326, to more than days for J Except for J849+58, for which the dataset does not cover a substantial number of activity cycles, the remaining three sources show intense variability at practically all frequencies, a behaviour commonly seen in blazars (e.g. Richards et al. 2; Aller et al. 2; Boettcher 22; Böttcher 24). The phenomenologies seen in these light curves vary significantly and mostly in terms of the observed amplitude of the different events. In these terms, J shows very weak outbursts. J shows at least two major events at the high-end of the bandpass at around 56 3 and 56 6, which disappear at lower frequencies. J shows frequent recursive events of activity that occur considerably fast; their characteristic times are of the order of 4 5 days at the highest frequencies (see Table B.). Most of the observed events can be identified at all frequencies (apart from the very low end of the bandpass) and with a time delay between frequencies (e.g. see event around 56 5 for J948+22), which is indicative of intense spectral evolution that is also routinely seen in blazar light curves (e.g. Rani et al. 23). The variability amplitude is frequency dependent, following the typical fashion: higher frequencies vary more. For J for example, the amplitude of the most prominent event is more than twice as large at 32. GHz as at 4.6 and more than six times lesser at.45 GHz; yet another indication that the variability mechanisms seem to be the same as those acting in typical blazars (e.g. Valtaoja et al. 992). Table 8 reports some characteristic parameters for these datasets. Specifically, for each frequency and source we report the total number of data points N, the dataset length Δt, the mean flux density S, the standard deviation around that mean σ, and the modulation index m. In the following we roughly describe the most noteworthy phenomenological characteristics of the light curves separately for each source. 4.. J The multi-frequency light curves of J are shown in Fig. 2. It shows a clearly different behaviour between high- and low-frequency bands despite the rather sparse sampling. Of the main events present in the and GHz light curves around 56 4, 56 3 and the first one disappears already at.45 GHz, the second is still present at 8.35 GHz, and the last can be traced down to 4.85 GHz. These events also show different characteristics such as rise and decay times. Generally, the source activity is very moderate at the lowest frequencies. The neighbouring frequencies show associated events and strong evidence of intense spectral evolution. Differences between similar frequencies can be regarded as unusual because the events disappear fast over frequency. At the highest frequencies, flux density variations by factors of about 5 or more are present, which are absent at the lowest frequencies. The spectral evolution may be an even better proxy of this acute behaviour, as we discuss later. A55, page 7 of 22

8 A&A 575, A55 (25) J324+34, H GHz 43. GHz GHz GHz J849+58, SBS GHz 43. GHz J324+34, H GHz 4.6 GHz 23.5 GHz J849+58, SBS GHz 4.6 GHz 23.5 GHz J324+34, H GHz 4.85 GHz 8.35 GHz J849+58, SBS GHz 4.85 GHz 8.35 GHz J324+34, H GHz 4.85 GHz 8.35 GHz.45 GHz 4.6 GHz 23.5 GHz 32. GHz 42. GHz GHz GHz J849+58, SBS GHz 4.85 GHz 8.35 GHz.45 GHz 4.6 GHz 23.5 GHz 32. GHz 42. GHz Fig. 2. Radio light curves available for J (left column) and J (right column) at all available frequencies. From top to bottom we present the light curves three different frequency bands: low: 2.64, 4.85 and 8.35 GHz, intermediate:.45, 4.6 and 23.5 GHz, and high: 32, 43.5, (when available) and GHz (when available). At the very bottom the datasets are shown over-plotted together for comparison. For the same reason, for each source and axes the boundaries are kept identical. Lines connecting the data points have been used everywhere to facilitate visual inspection. Each frequency is consistently represented by the same colour and symbol. Only data points with a S/N better than 3 have been used. A55, page 8 of 22

9 E. Angelakis et al.: Radio jet emission from GeV-emitting NLSys J GHz 43. GHz GHz GHz J55+326, PKS GHz 43. GHz J GHz 4.6 GHz 23.5 GHz J55+326, PKS GHz 4.6 GHz 23.5 GHz J GHz 4.85 GHz 8.35 GHz J55+326, PKS GHz 4.85 GHz 8.35 GHz J GHz 4.85 GHz 8.35 GHz.45 GHz 4.6 GHz 23.5 GHz 32. GHz 42. GHz GHz GHz J55+326, PKS GHz 4.85 GHz 8.35 GHz.45 GHz 4.6 GHz 23.5 GHz 32. GHz 42. GHz Fig. 3. Radio light curves available for J (left column) and J (right column) at all available frequencies similar as is Fig. 2. A55, page 9 of 22

10 A&A 575, A55 (25) Table 8. Summary of the mean flux densities and corresponding standard deviations for each observing frequency and source presented in Figs. 2 and 3. Source Observable Units J N Δt (days) S (Jy) σ (Jy) m (%) J N Δt (days) S (Jy) σ (Jy) m (%) J N Δt (days) S (Jy) σ (Jy) m (%) J N Δt (days) S (Jy) σ (Jy) m (%) Notes. For every entry we also report the number of data points N and the modulation index m = σ/ S as a measure of the apparent variability amplitude. Only data points meeting the condition of S/N 3 are included. Finally, we note that the baseline at all frequencies is practically flat, an indication that all the variability incidents evolve fast and dissipate in a dominating relic jet that either does not display any signs of variability or does so at a very slow pace; the dynamics of the radio SED shape point toward such an interpretation, as well J The very few available SEDs for this source indicate that this is another interesting and very active source. Unfortunately, the lack of a large enough and adequately sampled dataset prevents any systematic quantitative analysis. Nevertheless, the source shows activity cycles at all frequencies,which evolve systematically slower at lower frequencies (see the slow rising trend that becomes faster toward higher frequencies). It remains to be studied whether there is spectral evolution and what its characteristics are. Clearly, longer time-baselines are needed before any sensible results can be reached J J is among the best studied sources because it has been the first RL NLSy to be detected by Fermi. The light curves shown in Fig. 3 clearly indicate intense and repetitive variability that is prominent at all frequencies except possibly at 2.64 GHz, where most of the outbursting events have already smeared out. The light curves show variability corresponding to factors of more than 3 even at frequencies as low as.45 GHz or below. As an example, the light curve at 8.35 GHz undergoes a series of clearly discernible outbursts at s, approximately 55 2, 55 24, 55 49, , , and These events, which are single flares or sub-flares, appear in practically every densely enough sampled light curve except for that at 2.64 GHz (where the events have already disappeared). Even more interestingly, the moment of occurrence of an event appears at progressively later times as the frequency decreases, which indicates opacity effects. That is, a local maximum in a light curve corresponds to the instant at which the emitting plasma radiation becomes optically thin at the observing frequency. If the flare is associated with the emergence of an adiabatically expanding plasmon, this instant should indeed appear at progressively later times as the frequency drops. This claimed spectral evolution is very clearly seen in Fig. 4 as we discuss below. Finally, the pace at which the observed flux density (ds /dt) increases during a flare is clearly a function of frequency, with higher frequencies showing much larger derivatives J The last source in our list of targets is J Its mean flux density of 57 mjy at 4.85 GHz, makes it the brightest member of the sample. Significant variability is also present, but with qualitatively different characteristics. The most importantdifference at least compared with J is that the clear spectral evolution with the delay of the peak as a function of frequency is not as obvious here. Pairs of adjacent frequencies such as 4.85 and 8.35 GHz seem to show events in phase, although the lowest frequency, 2.64 GHz, does indeed lag by clearly discernible time spans. At intermediate frequencies for example at.45 GHz the amplitude of variability reaches moderate factors of.5. Finally, frequencies below.45 GHz show a long-term decaying trend modulated by the faster variability discussed above. 5. Flare decomposition: parametrising variability The parametrisation of flux density outbursts in terms of amplitude and time scale is commonly used as a method to constrain A55, page of 22

11 E. Angelakis et al.: Radio jet emission from GeV-emitting NLSys S (Jy) S (Jy) S (Jy). J J J the variability brightness temperature T var associated with the event, on the basis of causality arguments. The variability brightness temperature can only provide a lower limit of the intrinsic brightness temperature T B.ValuesofT var in excess of independently calculated limiting values of the brightness temperature are generally attributed to Doppler boosting, which provides a handle for the computing limiting Doppler factors, D. Assuming an equipartition brightness temperature upper limit of 5 K (Readhead 994), one can estimate the Doppler factors required to explain the observed excess. The combination of variability Doppler factors with Very Long Baseline Interferometry (VLBI) measurements of the apparent speeds allows computing the plasmoid bulk velocity and jet viewing angle (e.g. Lähteenmäki & Valtaoja 999). Here, we are equally interested in all these properties for our four RL NLSys, and most importantly, in investigating the possible differences in the characteristics of different flares in the same light curve, rather than retrieving the characteristics of an average behaviour. A practice commonly followed in variability studies is the implementation of time-series analysis methods that are designed to reveal such quantities; for example the structure function analysis (Simonetti et al. 985) and the discrete correlation function (Edelson & Krolik 988). One of the most important caveats of such methods, however, is that they are extremely sensitive to parameters that are difficult to determine in moderately sampled light curves such as the onset of a flaring episode or the shape of the temporal behaviour of the measured flux density. In fact, even minor changes in such parameters can result in differences in the estimation of the variability brightness temperature beyond an order of magnitude. Furthermore, in most cases, these tools are designed to detect a dominant behaviour that smears out possible significant differences in the characteristics of individual flares of the same source and even at different observing frequencies. To overcome such complications, we introduce a novel method for: a. first creating cumulative light curves by conveniently shifting and re-normalising the observed light curves. This operation is meant to highlight the flares that are detectable at a wide range of frequencies; b. and subsequently subjecting those light curves to all necessary operations to extract the desired parameters (i.e. flare onset, duration, amplitude, etc.). S (Jy) J Fig. 4. Radio SEDs of the four monitored NLSy galaxies. The data points are connected with straight segments to guide the eye. The legend denotes the fractional year for each SED The guiding principles while developing this approach were to:. avoid complications introduced by the superposition of simultaneously acting processes. For example, time-series analysis methods often return unrealistically long timescales only as the result of having for example a long-term almost-linear trend underlying much faster events. 2. To accommodate a generic approach in the treatment of every flare. That is, to parametrise each event independently (for each source and frequency) and investigate the possibility of different behaviours (and possibly variability mechanisms) acting in the same source at different times. For J324+34, for example, the most prominent event seems to demand such an approach because its phenomenology is very different fromthe rest (Fig. B.). All the details of the method are discussed in Appendix A. A55, page of 22